28 CHILDHOOD OF ANIMALS 



in many successive moults, until a moult comes after which the young 

 creature has the final form of its species. The seas teem with 

 these larvae, especially in summer, when the water is warm. They 

 feed on one another, and on the small floating plants which, like the 

 green herbage of the land, are the ultimate food-supply of the living 

 world, and they themselves are preyed upon by hosts of fishes. 

 The larvae appear in many curious shapes, but in those cases where 

 there are the greatest number of successive larvae and moults be- 

 tween the egg and the adult, the series shows a rough correspondence 

 with what may be supposed to be the ancestral history of the 

 crustacean in question. In those with fewer larvae the jumps are 

 bigger, some stages being suppressed, whilst the regularity of the 

 sequence is often confused by the premature appearance of some of 

 the organs or appendages, and the retarded appearance of others. 

 The starting-point in those larvae in which the series is most com- 

 plete, and which appears in more different kinds of Crustacea than 

 any other larva, is what is called the nauplius. The nauplius 

 (Fig. 14) has an oval body, not divided into rings or segments, 

 with a large median eye on the dorsal surface of the anterior end. 

 It has a mouth on the ventral surface, under the eye, protected by 

 a kind of membranous upper lip, and it has three pairs of swimming 

 appendages, the front pair of which occupy the position of, and 

 correspond with, the antennules or front pair of feelers of the lobster 

 or crayfish. Those of the second pair are forked, and usually have 

 hooks at their bases which lie on either side of the mouth and serve as 

 jaws. They correspond with the antennae, or second pair of feelers 

 of the adults. The third pair, situated a little farther back, are also 

 forked and correspond with the mandibles or true jaws of the adults. 

 In prawns (of the genus Penaus) the nauplius larva is succeeded 

 by a larger larva called the metanauplius (Fig. 14), in which the 

 swimming parts of the third pair of appendages are smaller whilst 

 a strong jaw portion is developed. Behind, there are the beginnings 

 of four other pairs of limbs. Next comes a protozoea larva with 

 the same seven pairs of appendages, a carapace or shell beginning to 

 spread over the dorsal surface of the anterior part of the body, 

 and a long, forked, but unjointed abdomen. The third pair of 

 appendages has ceased to be of use in swimming, and is wholly 

 transformed to the pair of jaws or mandibles. The paired eyes begin 

 to show through the carapace. For several successive moults 

 there is not much change in shape, but. the eyes push through the 

 carapace, and the abdomen becomes longer, is divided into joints, 



